How Do Radio Telescopes Function?


By Emily LevesqueUniversity of Washington

Grote Reber was 21 years old when he first read about Karl Jansky’s discovery of “star noise” in 1933. He immediately got excited at the possibilities. Reber set about building his own radio telescope in his backyard. Completed in 1937, Reber’s telescope had a parabolic 31-foot dish and a radio receiver mounted directly above it.

Radio telescopes with sunset sky in the background
Radio waves bounce off the metal surfaces of radio telescopes and gather into a receiver. (Image: Haizhen Du/Shutterstock)

Odd-looking Radio Telescopes

In 1938, Reber successfully reproduced Jansky’s observations of radio waves from the Milky Way. He went on to begin publishing in the Astrophysical Journal and then undertook the first radio wavelength survey of the sky. His published research finally kicked off a surge of interest in radio astronomy, and soon observatories were building radio telescopes of their own.

Grote Reber’s antenna may look a bit more recognizable than Karl Jansky’s, but radio telescopes may still look decidedly odd to someone used to thinking of telescopes as having shiny glass mirrors. Most of them aren’t ensconced in protective domes, and while there’s at least a parabolic dish that sounds at least vaguely similar to the parabolic primary mirrors of telescopes like the Hale 200-inch at Palomar, radio telescopes still look less like shiny reflective surfaces and more like giant metal bowls.

The trick comes when you remember that we’re looking at radio telescopes with our human, visible-light-detecting eyes. At the long wavelengths of radio light, invisible to our eyes, the metal surfaces of radio telescopes are shiny. Radio waves pouring down from the sky will bounce off of a parabolic radio dish and get focused into a receiver in the same way that visible light will get bounced off of a traditional mirror and focused into an eyepiece or camera.

Reber’s antenna
Grote Reber’s antenna was decidedly odd-looking when compared with the default image of what a telescope looked like. (Image: Grote Reber/Public domain)

Bigger Is Better

Radio telescopes also follow the same basic optics principles as visible-light telescopes. Making these telescopes big is still valuable. We may be working at much longer wavelengths, but the basic physics of astronomy still applies. If a telescope has a large collecting area, we can gather more light from faint and distant objects, and if a telescope has a large diameter, we can capture increasingly sharper images of the night sky.

Fortunately, although still challenging, when it comes to building enormous radio telescopes, it is at least a bit easier. You don’t need a massive glass blank for a mirror; instead, you can use thin sheets of metal to fashion a dish. Second, the precision of the dish doesn’t need to be quite as unforgivably perfect.

Under supervision and working carefully, radio telescopes can actually be walked on. Radio telescopes can also observe through all sorts of weather. They can operate day or night since the Sun doesn’t emit much light in the radio frequency, and continue to capture data even through clouds, fog, rain, and snowstorms. At some radio telescopes, snow will accumulate in the bowl of the dish, and it’s most easily cleared by sending an astronomer out onto the dish with a broom.

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Easier Said Than Done

The differences in engineering have made it possible to build some truly enormous radio telescopes. The largest fully steerable radio telescope in the world is the 100-meter Robert C. Byrd Green Bank Telescope, built in eastern West Virginia. A 100-meter telescope is already astounding, but radio telescopes can get larger still. 

Robert C. Byrd Green Bank Telescope
The 100-meter Robert C. Byrd Green Bank Telescope is the biggest fully steerable telescope in the world. (Image: Jet Propulsion Laboratory/Public domain)

One of the biggest and most famous is Arecibo Observatory, with a dish 1,000 feet across built into the natural topography of a sinkhole in Puerto Rico. Since the dish is built right on the ground, Arecibo can’t quite turn and point like a typical telescope, but by tipping its enormous receiver, hanging 500 feet above the dish, it can focus on and observe objects that pass overhead.

It might be tempting, looking at the large telescopes and 24-hour observing capabilities, to think of radio astronomy as easy. In truth, radio observations are some of the most technically challenging in all of astronomy, and effectively operating radio telescopes and detecting signals from space can be a tricky and delicate proposition.

The difficulty comes when considering what radio telescopes call noise. We already know that radio telescopes have different definitions of what we might consider a shiny mirror or a perfect parabola. As it turns out, radio telescopes also have a different definition of what we would consider dark.

What Is Dark for Radio Telescopes?

Most visible-light telescopes operate at night for obvious reasons: during the day the blinding light of the sun overwhelms the light of everything else in the sky, making it nearly impossible to detect the much fainter data from background stars or planets. Radio telescopes require darkness as well, but at very different wavelengths.

To pick up faint background signals from space, radio telescopes need to be protected from contaminating radio light sources, but this can be difficult since the walls of a typical observatory dome or building aren’t able to stop radio light. Instead, radio telescopes are placed in remote corners of the planet, not unlike the world’s best visible-light telescopes. 

Many observatories keep radio and optical telescopes on the same mountaintops for this very reason. Entire ranges of radio wavelengths are also protected by international regulations, restricting them from commercial or military use so that no Earth-born signals will interfere with radio astronomers’ science.

Common Questions about How Radio Telescopes Function

Q: Why do radio telescopes look like they don’t have shiny reflective surfaces?

Even though human eyes can’t see it, the metal bowls in radio telescopes are shiny reflective surfaces in the case of longer wavelengths like radio waves. When the radio waves hit the surface, they bounce off and get focused on a receiver just like a traditional mirror.

Q: Why is building larger radio telescopes easier than building large visible-light telescopes?

Radio telescopes don’t need a massive glass blank to be used as a mirror. Instead, thin sheets of metal can be used to make a disk. Also, radio telescopes don’t need to be as precise as visible-light telescopes.

Q: Why do radio telescopes have to be in remote places?

To be able to pick up background signals that are faint, radio telescopes have to be far from any contaminating radio waves on Earth. Since the walls of a radio observatory aren’t able to stop radio light, radio telescopes have to be placed at remote corners of the planet.

Keep Reading
The History of the Telescope
George Hale: The Maker of Massive Telescopes
Exciting Developments in Telescope Building